A conventionally known gas sensor is configured such that a closed-bottomed tubular gas detection element (to-be-held member) is held in a tubular metallic shell. An example of such a gas sensor is shown in FIG. 13, which shows a partial cross section of a gas sensor. A gas sensor 901 shown in FIG. 13 is an oxygen sensor attached to an exhaust gas pipe of an internal combustion engine and adapted to measure oxygen concentration in exhaust gas. The gas sensor 901 includes a closed-bottomed tubular gas detection element 911, whose distal end (a lower end in FIG. 13) along the direction of an axis C is closed, and a tubular metallic shell 931, which coaxially holds the gas detection element 911 therein.
The gas detection element 911 includes a projection 913, which is circumferentially formed at the central portion of the gas detection element 911 with respect to the direction of the axis C and projects radially outward. The projection 913 has a first tapered outer circumferential surface 913t1 (also referred to as a “distal end surface”) which is located on its distal end and whose diameter increases from its distal end side toward its proximal end side, a second tapered outer circumferential surface 913t2 (also referred to as a “proximal end surface”) which is located on its proximal end and whose diameter increases from its proximal end side toward its distal end side, and a central outer cylindrical surface 913m extending therebetween. The gas detection element 911 is formed from an oxygen-ion-conductive solid electrolyte. The gas detection element 911 has an inner electrode 915 cladding an inner circumferential surface 911n, and an outer electrode 917 cladding an outer circumferential surface 911m. 
The metallic shell 931 includes a distal end section 933 (a lower section in FIG. 13), a central section 935, and a proximal section 937 (an upper section in FIG. 13). A through-hole whose wall is an inner circumferential surface 931n extends through the metallic shell 931, and its diameter reduces from the side toward the proximal end to the side toward the distal end.
The distal end section 933 has an inner circumferential surface 933n having a relatively small diameter, and a male-threaded portion 933g formed on its outer circumference and adapted to attach the gas sensor 901 to the exhaust gas pipe. A protection cap 951 is attached to a distal end portion of the distal end section 933 for the purpose of protecting a distal end section of the gas detection element 911. The protection cap 951 assumes a closed-bottomed tubular shape and has a number of gas introduction holes 951k for introducing exhaust gas into the interior of the gas sensor 901 from the exhaust pipe. A gasket 953 is attached to a proximal end portion of the distal end section 933.
The central section 935 is composed of a stepped portion 935b having a tapered inner circumferential surface 935t1 (also referred to as a “support surface”), which is connected with the inner circumferential surface 933n of the distal end section 933 and whose diameter increases toward the proximal end side of the gas sensor; a tubular portion 935c having a central inner circumferential surface 935n, which is connected with the tapered inner circumferential surface 935t1 and which has a diameter larger than that of the inner circumferential surface 933n. A radially outer portion of the central section 935 is formed into a hexagonal flange portion (a tool engagement portion) 935r, which is used in attaching the gas sensor 901 to the exhaust gas pipe.
The proximal end section 937 has an inner circumferential surface 937n, which is connected with the central inner circumferential surface 935n of the central section 935 and which has a diameter greater than that of the central inner circumferential surface 935n. 
An annular plate packing 957 of metal is disposed on the tapered inner circumferential surface 935t1 of the central section 935 of the metallic shell 931. The first tapered outer circumferential surface 913t1 of the projection 913 of the gas detection element 911, which is coaxially inserted into the metallic shell 931, abuts the plate packing 957. In other words, the stepped portion 935b of the central section 935 of the metallic shell 931 and the projection 913 of the gas detection element 911 are engaged via the plate packing 957. Since the outer electrode 917 is also formed on the projection 913, the metallic shell 931 and the outer electrode 917 of the gas detection element 911 are electrically connected via the plate packing 957.
A C-type first wire packing 959 is disposed in such a manner as to abut the second tapered outer circumferential surface 913t2 of the projection 913 of the inserted gas detection element 911 and the inner circumferential surface 931n (the inner circumferential surface 937n of the proximal end section 937) of the metallic shell 931.
In a region located toward the proximal end of the gas sensor with respect to the first wire packing 959, a powder is charged into an annular clearance provided between the outer circumferential surface 911m of a proximal end portion of the gas detection element 911 and the inner circumferential surface 931n (the inner circumferential surface 937n of the proximal end section 937) of the metallic shell 931, thereby forming a charged seal layer 961.
In a region located toward the proximal end of the gas sensor with respect to the charged seal layer 961, a distal end section 973 of a sleeve 971 is inserted into an annular clearance provided between the outer circumferential surface 911m of the gas detection element 911 and the inner circumferential surface 931n (the inner circumferential surface 937n of the proximal end section 937) of the metallic shell 931. The distal end section 973 of the sleeve 971 assumes the form of a circumferential projection projecting radially outward and has a tapered outer circumferential surface 973m whose diameter increases toward the distal end side of the gas sensor. A C-type second wire packing 965 is disposed on the tapered outer circumferential surface 973m. The distal end of the distal end section 937 of the metallic shell 931 is bent radially inward in such a manner as to cover the second wire packing 965, thereby compressing the second wire packing 965 by means of crimping. The compressive crimping action also axially compresses the first wire packing 959 and the charged seal layer 961. As a result, the first wire packing 959 is elastically deformed. An elastic force induced by the elastic deformation coaxially holds the gas detection element 911 in the metallic shell 931.
An element-side terminal 981 is inserted into the gas detection element 911 and electrically connected with the inner electrode 915 of the gas detection element 911.
A Document related to the above technique is disclosed in, for example, Patent Document 1.    Patent Document 1: Japanese Utility Model Application Laid-Open (kokai) No. 53-95884